Conventionally, a semiconductor device has a structure as described below. Here, a photodetector device having a semiconductor element such as a photodiode is taken as an example.
As shown in FIG. 4, the photodetector device is composed of a common lead 31 having at one end an approximately square-shaped island 31a, specific leads 32 and 33 formed on both sides of and approximately parallel with the common lead 31, a semiconductor element 34 die-bonded on the island 31a, a plurality of wires 35 made of gold or other for electrically connecting the semiconductor element 34 to the top surface of the island 31a and to the specific leads 32 and 33 by wire bonding, and an approximately square-shaped resin seal 36 made of epoxy resin for sealing in the semiconductor element 34 and the wires 35 (the resin seal 36 actually is opaque, though it is shown as transparent in FIG. 4). On the top surface of the above semiconductor element 34 are formed a plurality of projection-like electrode pads (not shown) made of aluminum or other. On these electrode pads are bonded wires 35, of which four are, at the other end, bonded on the top surface of the island 31a and two are, at the other end, bonded on the specific leads 32 and 33, respectively. The first four wires are electrically connected to the island 31a so as to be grounded, and thus serve to adjust the resistance against a current flowing between the specific leads 32 and 33.
The resin seal 36 is so formed as to seal in the semiconductor element 34 and the wires 35 as shown in FIG. 5 by use of a resin-forming mold (not shown). The resin seal 36 has, at its portion approximately above the semiconductor device 34, a hemispherical condenser lens 37 for leading light from outside to the semiconductor element 34.
However, this photodetector has been suffering from a following problem.
The above resin seal 36 is heated to seal in the semiconductor element 34 and the wires 35, and thereafter, as the resin seal 36 cools down, it develops contraction stress inside itself. This causes the wires 35 to be pulled together with the resin, of which the resin seal 36 is made, in the direction of the inward contraction of the resin. On the other hand, the island 31a, on which one end of each wire 35 is stitch-bonded, contracts far less than the resin seal 36 because of their different thermal expansion coefficients.
As a result, the wires 35 are subjected to force that pulls them in the direction of the contraction of the resin and that tends to tear the ends of the wires 35 off the island 31a, and eventually the wires 35 are broken as shown in FIG. 6. Moreover, the same can be caused even by variations in ambient temperature.
An object of the present invention is to provide a highly reliable semiconductor device that is substantially free from wire breakage.